1. Field of the Invention
The invention relates to an electric power steering system.
2. Description of Related Art
In an electric power steering system in which power output from an electric motor is transmitted to a steering mechanism via a reduction mechanism, a worm reduction mechanism may be used as the reduction mechanism (for example, see FIG. 8 in Japanese Patent Application Publication No. 2004-232675 (JP-A-2004-232675)). A worm shaft of the worm reduction mechanism shown in FIG. 8 of JP-A-2004-232675 is coupled to a rotary shaft of the electric motor. Both end portions of the worm shaft are respectively supported by a housing via bearings. Because axial internal clearances of the bearings are made large, the worm shaft is movable in the axial direction.
With the above configuration, when the electric motor is not driven, for example, at the moment at which steering of a steering wheel is started, the worm shaft is movable in the axial direction by rotational force transmitted from the steering wheel to a worm wheel. Thus, it is possible to reduce a driver's steering effort when the electric motor is not driven.
However, in a state where the internal clearance of each bearing is large, when the worm shaft is displaced in the axial direction, an inner ring of each bearing is also displaced together in the axial direction, and balls of each bearing collide with the inner ring or an outer ring to generate collision noise. On the other hand, when the internal clearance of each bearing is small, displacement of the worm shaft is inhibited. Specifically, when the internal clearance of each bearing is small, an end portion of the worm shaft, which is distant from the motor, cannot be displaced toward the worm wheel. Therefore, when a tooth portion of the worm shaft abrades, it is difficult to close up a backlash between the worm shaft and the worm wheel. Therefore, there occurs collision noise resulting from a collision at the time of meshing between the worm shaft and the worm wheel.
Thus, the axial internal clearance of each bearing should be neither too large nor too small. Therefore, strict dimensional management of each bearing is required. Such dimensional management leads to an increase in manufacturing cost.